190 million years ago, there were no mammals on Earth. Instead, there were furry, mouse-like creatures no bigger than a paperclip. And they suddenly began to grow brains 10 times larger than their relative body size. What happened?
The story of the mammalian brain's rapid growth is well-known to paleontologists. The question is, which parts of the brain began to grow first? Today in the journal Science, paleontologist Timothy Rowe and colleagues argue that the size of our brains today can be traced back to evolutionary leaps in our tactile senses and - most importantly - a highly-developed sense of smell.
Rowe and his team used 3D imaging technology to create brain models for two proto-mammals, called Morganuocodon and Hadrocodium. The scientists found the proto-mammals' skulls had changed shape over time to admit larger olfactory bulbs, or the regions of the brain devoted to scent. Plus, the creatures' fur would have enhanced their abilities to feel the world, via nerves activated by each hair.
Morganuocodon and Hadrocodium had brains at least fifty percent larger than those of their immediate precursors on the evolutionary tree.
Rowe and colleagues write in Science today:
At its start, the brain in the ancestral mammal differed from even its closest extinct relatives specifically in its degree of high-resolution olfaction, as it exploited a world of information dominated to an unprecedented degree by odors and scents.
In other words, mammals evolved to smell and feel the world better than other animals. This expanded their brains considerably, and quite rapidly in evolutionary terms.
It seems that developing new or better senses is often connected to rapid evolution. A scientist who studies elephantfish recently showed that speciation - splitting into two or more species - occurs in fish when their brains begin to evolve better sensory perception.